This invention relates generally to an apparatus for the generation of electrical power at a very small scale and, in particular, to thermoelectric generators based on counterflow heat exchanger combustors.
It is well known that the use of combustion processes for electrical power generation provides enormous advantages over batteries in terms of energy storage per unit mass and in terms of power generation per unit volume, even when the conversion efficiency in the combustion process from thermal energy to electrical energy is taken into account. For example, hydrocarbon fuels provide an energy storage density between 40 and 50 MJ/kg, whereas even modem lithium ion batteries provide only 0.4 MJ/kg. Thus, even at 5% conversion efficiency from thermal to electrical energy, hydrocarbon fuels provide 5 times higher energy storage density than batteries. Also, the waste products are primarily carbon dioxide and water as compared to toxic metals in the case of batteries. Furthermore, the power generation rate per unit mass or volume of combustion device is orders of magnitude larger than other technologies using chemical reactions, for example, fuel cells. Also, hydrocarbon fuels are relatively inexpensive, readily available, easily stored, and have much longer shelf lives than batteries. Furthermore, combustion-driven devices can use a variety of conventional hydrocarbon fuels without any pre-processing.
Despite these advantages, combustion processes for converting fuel energy to electrical energy have not yet proved practical for powering small scale devices such as cell phones and other portable electronics, which currently rely on batteries. Most approaches to combustion at a small scale use scaled-down versions of macroscopic combustion engines, for example, micro-scale rotary (Wankel) engines, free-piston engines, and micro gas turbine engines. However, when applied at a micro scale, these approaches have numerous difficulties. One of the most fundamental problems limiting combustion at a micro scale is flame quenching due to heat losses when the dimensions of the combustion chamber are too small. For stoichiometric hydrocarbon-air mixtures at atmospheric pressure, the minimum combustion chamber dimension in which flames can exist is about 2 mm when chamber walls are at atmospheric pressure. Furthermore, even if flame quenching does not occur, heat and friction losses become increasingly important at smaller scales since the heat release due to combustion and thus power output scales with engine volume whereas heat and friction losses scale with surface area.
Thus, there remains a need for an efficient microscale combustor that will enable combustion processes to be used for the generation of electrical power for small-scale low-power systems such as micro-electro-mechanical-systems (MEMS) devices, and portable electronic devices such as personal organizers, laptop computers, and wireless phones.
A generally toroidal counterflow heat exchanger forms the basis of a combustor that operates at a micro scale. An electrical microgenerator is similar and also includes a thermoelectric active wall composed of thermoelectric elements as part of a channel wall of the microcombustor. The heat exchanger is configured such that in a cross section parallel to an axis of rotation of the toroid surface, there is a central combustion region with openings to a reactant gas channel and an exhaust gas channel. The reactant channel and exhaust channel are coiled around each other and separated by a channel wall such that when the generator is operating, one surface of the channel wall in the interior of the heat exchanger is in contact with reactant gas and the other surface is in contact with exhaust gas. The counterflow arrangement reduces heat loss from the combustor and preheats the reactant gases. The microcombustor may have applications other than generating electrical current where very small amounts of heat are needed.
The microcombustor and microgenerator have an outer diameter of the toroid that is between about 2 and about 15 mm. Their height is between about 1 and about 6 mm. The central combustion region has a characteristic dimension less than about 1 mm.
The microgenerator includes a reactant gas inlet on the exterior of the heat exchanger in communication with the reactant gas channel and an exhaust gas port on the exterior of the heat exchanger in communication with the exhaust gas channel. An ignitor may be included within the heat exchanger. The thermoelectric material is electrically connected to the exterior of the microgenerator. The structural material of which the microgenerator is composed is a conductive metal. Electrodeposited platinum is a useful structural material. In some arrangements the structural material of the microgenerator provides the electrical connection between the thermoelectric elements and the exterior of the microgenerator.
The thermoelectric active wall consists of elements of n-type conductivity thermoelectric material and of p-type conductivity thermoelectric material and fins composed of thermally and electrically conductive material. The fins are configured to increase the temperature differential across the thermoelectric elements relative to the temperature difference between the thermoelectric elements and the reactant and exhaust gases. The fins may be T-shaped having a base portion and a top portion and arranged such that each base portion separates an n-type element and p-type element and the top portions extend into the reactant channel and into the exhaust channel. According to an aspect of the present invention the top portion of the fins is at least three times longer than the base portion of the fins. The invention additionally includes L-shaped fins and asymmetric T-shaped fins.
The microgenerator may include a partition wall juxtaposed between partial toroidal portions of the heat exchanger. In some arrangements, an igniter within the heat exchanger includes resistive elements or sparking elements which are connected to an external power source, optionally through conductors in the partition wall.
According to another aspect of the present invention, a method of fabricating a thermoelectric microgenerator or microcombustor based on electrodeposition of multiple layers of material is provided. The method includes electrochemically depositing a sacrificial material using a mask comprising an elastomer affixed to an electrode, electrochemically depositing a structural material using a mask comprising an elastomer affixed to a support and a separate electrode, and blanket electrodepositing a thermoelectric material without use of a mask. After all layers have been deposited, the structural material is removed to form the generator. Alternatively, the order in which the structural material and the thermoelectric material are deposited can be reversed. In another alternative process, each deposition of thermoelectric material is preceded by deposition of a thin barrier layer to overcome any incompatibility between the sacrificial material and the thermoelectric material. The invention also includes an anodeless mask for selectively electrodepositing material, the mask comprising a patterned elastomer affixed to a perforated non-electrode support composed of non-porous material.